As most of the liquid crystal display according to prior art utilizes signal transmission of Point to Point, the sequence controller (TCON) codes the control signal, such as the data source row latch signal (TP) and the signal (POL) controlling polarity reversal of the pixel voltage, and embeds the same in the differential signal to be transmitted from the first circuit board, such as C-board to the source driving circuit (source IC) through the second circuit board, such as X-board. The Source IC receives and decodes the differential signal and performs the subsequent signal process on demand. If it is going to measure the TP and POL signals after being decoded, the Source IC decodes and restores the control signal to be outputted to the second circuit board in a way of transistor-transistor logic voltage level for being employed to test the waveform and the sequence of the control signal.
However, after the control signal outputted by TCON has been through the subsequent code and decode processes, they cannot be directly measured on the first circuit board with the test apparatus. Thus, without the Source IC, the waveform and the sequence of the signal cannot be determined to be correct as modulating the initial control signal of TCON. Besides, as the image of the liquid crystal display appears to be abnormal, it is impossible to measure the waveform and the sequence of the control signal outputted by TCON in real time to determine whether they are correct, either. Certainly, there is no way to ensure whether the control signal outputted by TCON satisfies the requirement of the product or not.